Yesterday, a user mentioned that their BMC product cracked before ejection during injection molding. This cracking did not occur during production at an outsourced factory, but appeared after production resumed. Due to equipment changes, cause needs to be identified and resolved. User mentioned that changes in curing time did not improve situation; increasing mold temperature would improve it, but other factors are still being verified. Therefore, proposed solution is to strengthen cracked area.

 

This article will explain impact of mold temperature and curing time on product quality, providing injection molding personnel with a deeper understanding of BMC material processing.

Mold temperature is one of the most critical process parameters in BMC molding. 170-200℃ is a typical processing range, with specific temperature depending on specific resin system of BMC formulation (e.g., unsaturated polyester, vinyl ester, etc.).
Mold temperature primarily affects following aspects:
1. Curing Reaction Degree and Final Performance
Insufficient Temperature (e.g., below 170℃):
Incomplete Curing: Cross-linking reaction of resin cannot proceed fully, resulting in unreacted groups within product.
Poor Mechanical Strength: Product's flexural strength, impact strength, and hardness, among other mechanical properties, fail to meet design values.
Decreased Thermal Properties: Heat distortion temperature (HDT) decreases, making product prone to softening and deformation at high temperatures.
Poor Chemical Resistance: Due to incomplete cross-linking network, product is more susceptible to chemical corrosion.
Poor Surface Quality: Poor flowability can lead to low surface gloss, flow marks, shrinkage marks, or incomplete filling.
Insufficient Temperature (e.g., exceeding 200℃ or even higher):
Excessively Fast Curing: Resin reacts rapidly upon contact with mold surface, causing a sharp decrease in flowability.
Difficulty Filling/Scorching: Material may fail to fill mold cavity, or it may decompose or scorch (yellowing or black spots) at flow end due to a violent reaction.
Increased internal stress: Violent and uneven curing reactions can cause molecular chains to be "frozen" in a stress state, resulting in high internal stress, easy warping and deformation of product.
Decomposition risk: Some components may undergo thermal decomposition at excessively high temperatures, releasing gases, producing bubbles or voids, and reducing product strength.
Suitable temperature (within a reasonable range of 170-200℃):
Sufficient and controllable curing: Resin can undergo cross-linking reactions at an appropriate rate, forming a dense, uniform three-dimensional network structure.
Optimal mechanical properties: Product's strength, hardness, and toughness are fully utilized.
Good surface quality: Good material flowability, perfectly replicating surface gloss of mold cavity.
2. Flowability and Filling
Mold temperature directly affects viscosity of BMC. Higher temperatures result in lower initial resin viscosity and better flowability, which is beneficial for filling thin-walled or complex structures. However, as mentioned above, excessively high temperatures can negate this advantage due to excessively rapid reactions, and may even lead to premature loss of flowability.
3. Molding Efficiency
The higher mold temperature, the faster curing reaction rate, theoretically the shorter required curing time, the shorter molding cycle, and the higher production efficiency. However, this must be achieved while ensuring quality.

Curing time refers to the time material is held under pressure and reacts within mold cavity at a set mold temperature.
1. Insufficient Curing Time
Product Quality: This is the most direct impact. When product is demolded, exterior may be hardened, but interior may still be in an "uncured" state (under-cured). This will lead to:
Severely Insufficient Strength: Product is soft, with extremely poor mechanical properties, and is easily damaged during demolding or subsequent handling.
Dimensional Instability: After demolding, unreacted parts inside will continue to react, causing unpredictable changes in product dimensions (post-shrinkage, warping).
High Internal Stress: Uneven shrinkage between the incompletely cured inner layer and cured surface layer generates enormous internal stress, which can easily cause product to crack during storage or use.
Surface blistering/bulging: Unreacted components inside may vaporize upon subsequent heating, causing blistering on product surface.
2. Excessive curing time
Product quality: While more complete curing is desirable, it also has negative impacts:
Low production efficiency: Unnecessarily extended cycle times.
Increased energy consumption.
Potential over-curing risk: For some materials, excessively long curing times (especially at high temperatures) may lead to polymer chain degradation, making material brittle and reducing impact strength.
Internal stress: Under prolonged high temperature and pressure, material's thermal expansion and contraction are restricted by mold, potentially "locking in" some stress. Simultaneously, excessively long curing times may exacerbate difference in thermal expansion between material and metal insert, increasing internal stress.
3. Appropriate curing time
Uniform and complete curing inside and out.
Optimal mechanical strength, hardness, and thermal properties.
Minimized internal stress and best dimensional stability.
Achieving maximum production efficiency while ensuring quality.
Relationship between Curing Time and Mold Temperature: The two are closely related, following general laws of chemical reactions—Arrhenius equation. Generally, for every 10℃ increase in mold temperature, curing reaction rate approximately doubles, thus significantly shortening required curing time. For example, a product that requires 60 seconds to cure at 180℃ may only require 40-45 seconds at 190℃.

 

Setting a suitable mold temperature is a systematic process that requires comprehensive consideration of materials, product, mold, and production goals. Following are detailed steps and principles:
1. Refer to Material Supplier's Recommendations
This is the first and most important step. BMC suppliers will provide a recommended processing temperature range and typical curing time. Use this as initial benchmark.
2. Analyze Product Structure
Wall Thickness: Thick-walled products require slightly lower mold temperatures to prevent excessively rapid external curing and formation of shrinkage cavities or bubbles internally. Thin-walled products require higher mold temperatures to ensure fluidity and rapid curing.
Complexity: Products with complex structures and many ribs require good fluidity, can appropriately use upper-middle temperature range.
Installation or Absence: For products with metal inserts, difference in thermal expansion coefficients between inserts and BMC must be considered. Sometimes, a slight reduction in mold temperature is needed to reduce internal stress generated after cooling.
3. Determine Production Goals
Quality or Efficiency? During development phase, quality should be prioritized, and a mid-range temperature should be selected for testing. During mass production, while ensuring quality, temperature can be appropriately increased within permissible limits to shorten cycle time.
4. Process Debugging (DOE Method)
Select an initial value: For example, within recommended range (170-200℃), choose 185℃ as starting point.
Fix other parameters: Keep injection speed, pressure, holding pressure, etc., constant.
Conduct temperature gradient tests: Test at 180℃, 185℃, 190℃, and 195℃ respectively.
Evaluation and Optimization:
Appearance Inspection: Observe whether product surface is smooth, and whether there is scorching, material shortage, or flow marks.
Strength Testing: Perform mechanical property tests on the sample (e.g., flexural strength, Barcol hardness).
Dimensional Measurement: Check whether critical dimensions are stable and whether there is warping or deformation.
Internal Stress Assessment: This can be assessed by observing for cracks using solvent immersion (e.g., with ethyl acetate) or through thermal cycling tests.
Determine optimal curing time: At each temperature, gradually reduce curing time until product is just fully cured and meets performance standards. This point in time is minimum safe curing time at that temperature.
5. Consider Mold Temperature Control
Ensuring uniform and stable mold temperature is crucial. Temperature difference between different areas of mold should be controlled within ±5℃. Using a mold temperature controller and optimizing cooling channel design are key to achieving this. Uneven temperature directly leads to product warping, internal stress concentration, and uneven curing.